This is a cross-sectional study that examines immunologic parameters in children who have undergone lung transplantation and who are presenting with their first acute rejection episode or who are presenting with chronic graft rejection manifesting as bronchiolitis obliterans. Samples from these patients, including bronchioalveolar lavage (BAL) washings and lung biopsy material, will be examined for the phenotype of their respective lymphocyte populations, their activation status, and their lymphokine production profile.